In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more-complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. In any event, the hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2150° F.
Many approaches have been used to increase the operating temperature limits of the airfoil portions of gas turbine blades and vanes to their current levels. For example, the composition and processing of the base materials themselves have been improved, and a variety of solidification techniques have been developed to take advantage of oriented grain structures and single-crystal structures.
Physical cooling techniques may also be used. In one technique, internal cooling passages are present in the interior of the gas turbine airfoil. Air is forced through the cooling passages and out cooling openings at the external surface of the airfoil, removing heat from the interior of the airfoil and, in some cases, providing a boundary layer of cooling air at the surface of the airfoil.
The surfaces of the internal cooling passages may be protected with a diffusion aluminide coating, which oxidizes to an aluminum oxide protective scale that inhibits further oxidation and hot corrosion of the internal surfaces. A number of techniques for applying the internal diffusion aluminide coating are known, including chemical vapor deposition, vapor-phase aluminiding, and above-the-pack techniques. These approaches have the drawback that they also coat other exposed surfaces. Surfaces which are not to be coated may sometimes be protected by masking, but masking may not be practical in many circumstances. They also may require extensive plumbing arrangements in some cases to conduct the aluminiding gas to the internal surface to be coated, and in some cases the required plumbing arrangement cannot be achieved.
In another technique, a coating slurry containing a source of aluminum, a halide activator, and an oxide dispersant is applied to the internal surface. The slurry coating is chemically reacted to deposit aluminum on the internal surface. Slurry coating has the advantage that the spatial extent of the aluminum-containing coating may be limited to specific areas such as the internal surfaces. However, existing slurry-coating techniques have the drawback that they may leave undesirable contamination on the internal article surface of the blade in the form of residual components of the coating slurry. The residual components may be cleaned out of the narrow passages only with great difficulty, using high-pressure water jets, ultrasonic energy, and the like.
There is therefore a need for an improved approach to the depositing of aluminum-containing coatings on specific areas of surfaces, particularly the internal surfaces of articles such as gas turbine airfoils. The present invention fulfills this need, and further provides related advantages.